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Mixture expected to evaporate, but environmentalist concerned it could hurt marine life

The leak is at an oil and gas platform 74 miles southeast of Louisiana

A natural gas leak in the Gulf of Mexico left a four-mile wide "rainbow sheen" on the water's surface south of Louisiana, the Coast Guard said, but the owner of the well said it expects the leak to be plugged sometime Wednesday.

Houston-based Talos Energy said on Tuesday the gas flowed from a well that it was in the process of abandoning. The leak occurred while it was trying to permanently plug the well, located about 74 miles southeast of Port Fourchon, Louisiana.

Talos said it evacuated all five staff members from the platform and shut down the two other working wells there. It notified the Bureau of Safety and Environmental Enforcement and the Coast Guard and began a spill response.

Along with the gas (escaping into the air), the well leaked about six barrels of a liquid form of raw natural gas, or about 252 gallons, the company said. It expects the mixture to evaporate quickly.

The Coast Guard and federal environmental officials flew over the leak and found natural gas still flowing from the well, with a rainbow sheen visible on the surface measuring more than four miles wide by three-quarters of a mile long, the Coast Guard said.

The wellhead is on a platform above the water's surface, according to a spokesman for Talos Energy.

There is a concern that the gas leak could have a toxic effect on marine life.

"Toxic gases will damage the bodies of fish that come into contact by damaging their gills and causing internal damage," said Jonathan Henderson of the Gulf Restoration Network, an environmental advocacy group in New Orleans. "Marine species in the Gulf are more vulnerable when water temperatures are high and when oxygen concentrations are low like they are now."

Talos said the well is older and in a field developed in the 1970s. By 1998, the well was producing mostly water at a low-flowing pressure, so the company was plugging and abandoning it.

The company said it believes the age of the tubing may have contributed to the leak, though the Coast Guard said the cause is still under investigation.

The Coast Guard said the well is owned by Energy Resource Technology Gulf of Mexico. Talos acquired the company earlier this year.